This invention relates generally to implementation of holography, and more specifically to the replication of holograms by embossing.
Mass replicated holograms are commonplace, appearing on credit cards, consumer packaging, and the like. They typically serve as security devices as well as decorations. The holograms allow viewing of images formed in reflective light at locations other than the hologram surface. This can be in the form of a three-dimensional image of an object or of a complex image having different planes.
Such holograms are made by interfering two beams of coherent light at a finite angle with each other on a photosensitive medium. One of the beams interacts with an object whose image is to be recorded, and the other is a reference beam. An image of the object is focused into or near the surface of the resulting hologram by appropriate optical elements, which may include use of another, intermediate hologram. The resulting master hologram is made to be of surface relief type; that is, the image information is stored in surface variations.
The fragile photographic hologram master is then used to make a rigid embossing master plate in order to emboss replicas. Such a rigid master is formed in an electrolytic bath on the surface of the photographic master. Multiple masters are then made from this first master, for use in embossing holograms on substrate material. The metal masters are generally made of nickel.
The most common substrate materials into which holograms are embossed by such masters include Mylar, a polyester, or similar sheet plastic material. Usually, such material has been coated with a thin layer of reflective material, such as aluminum, in order to result in a reflective hologram. The aluminum is usually vacuum deposited onto the substrate in a thickness merely sufficient to provide reflective properties to the substrate.
The embossing process involves heating the aluminized substrate to a temperature at which it becomes plastic, and then the nickel master plate is forced against the substrate to imprint the surface relief pattern in it. The embossing is usually done through the aluminum coating, but it is the substrate itself that, upon cooling, holds the surface relief pattern. The aluminum layer follows the surface relief pattern and thus is capable of reconstructing the original holographically recorded image in reflected light.
In actual use, these replicated holograms are usually subsequently attached to another surface, such as that of a credit card blank. Such attachment is made by use of an ordinary adhesive or by use of hot stamping techniques.
However, the practice of affixing hologram decals onto other articles may not be most desirable for decorative or security considerations. Often it is more elegant and more convenient to emboss the hologram directly onto the material forming the article. Also, a hologram formed directly on the material to be authenticated makes a better security device.
With metallic materials, the relative hardness makes embossing very difficult if not impossible. Nevertheless, U.S. Pat. No. 4,725,111, assigned to the same assignee of the present application, discloses a method of embossing hologram directly onto aluminum sheet or can. This is possible because above certain elevated temperatures, the hardness of aluminum is reduced to much below that of the nickel master.
It is a primary object of the present invention to develop techniques for embossing holograms onto a broader range of substrates.
It is another object of the present invention to develop techniques for forming holograms on substrates with hardness even comparable to or much greater than that of the metal master.